A conventional flat umbrella as shown in FIG. 1 includes six ribs R pivotally secured to an upper notch or lower runner which is formed as a flat rectangular shape with each rib R pivotally held in each pivotal socket N1 and with two opposite side ribs R opened to be limited by a retarding portion L formed on a front or rear wide side portion of the upper notch or lower runner. Since the two ribs as spaced by the retarding portion L at the wide side portion are separated at a big angle (90 degrees), as compared to other smaller angles each defined between every two neighboring ribs R at the narrow side portion, it will influence the smooth folding operation and require further inconvenient pleating of the umbrella cloth as disposed on the unevenly spaced ribs when closing the umbrella. Meanwhile, the six ribs R should support the whole umbrella cloth, their strength is not enough durable for operating the umbrella.
The present inventor has found the drawbacks of the conventional flat umbrella and invented the present umbrella as flattened and shortened.
The object of the present invention is to provide an umbrella including: a flat upper notch formed on a top of a central shaft and a flat lower runner slidably held on the central shaft for pivotally securing a plurality of ribs between the upper notch and the lower runner, either the upper notch or lower runner having four ribs pivotally secured to a narrow right side portion of the upper notch or the lower runner, and having three ribs pivotally secured to a narrow left side portion of the upper notch or the lower runner, thereby decreasing the separating angle between every two opposite ribs disposed on a wide front or rear side portion of the upper notch or the lower runner for smoothening the folding operation when closing the umbrella and also for enhancing the strength of the umbrella ribs.